High-speed imaging devices (high-speed video cameras) used for taking consecutive images of high-speed phenomena, such as types of explosions, destructions, combustions, collisions or discharges, for only a short period of time, have been conventionally developed (for example, refer to Non-Patent Document 1). Such high-speed imaging devices need to perform an ultrahigh-speed imaging operation at a level of one million frames per second or even higher. Accordingly, they use solid-state image sensors capable of high-speed operations, which have special structures different from those of conventional image sensors used in normal video cameras, digital cameras and similar devices. As one example of this type of solid-state image sensor, a device called the “in-situ storage image sensor” is disclosed in Patent Document 1 and other documents. This image sensor has a storage charge-coupled device (CCD) capable of consecutively recording a predetermined number of frames without outputting signals to the outside of the sensor. The applicant of the present patent application proposed, in Patent Document 2 and other documents, a CMOS-structure image sensor having a capacitor capable of consecutively recording a predetermined number of frames of image signals without outputting the signals to the outside of the sensor.
In these types of solid-state image sensors, an embedded photodiode is normally used for photoelectric conversion. In the aforementioned ultrahigh-speed imaging operation, the exposure time for the photodiode is extremely short as compared to the normal imaging. Therefore, in order to ensure high detection sensitivity, it is necessary to make the largest possible amount of light fall onto the photodiode in each pixel. For this purpose, it is desirable to make the light-receiving surface of the photodiode as large as possible. However, when the light-receiving surface of an embedded photodiode having a commonly known structure is enlarged, the time required for the photocharges created in the photodiode to reach the floating diffusion region will be measurable, so that a portion of the photocharges fails to reach the floating diffusion region within the short period of time allotted for the photoelectric conversion. As a result, a portion of the charges that should be read out in the current frame will be read out in the next frame. Thus, an afterimage is formed. This is one of the major causes of image degradation in a solid-state image sensor designed for ultrahigh speed operations.
To address this problem, in the solid-state image sensors described in Patent Documents 3 and 4 (as well as other documents), the floating diffusion region is provided at or near the center of the rectangular light-receiving surface of the embedded photodetector, and the gate electrode of the transfer transistor is disposed around the floating diffusion region. This arrangement reduces the maximum distance from the light-receiving surface to the floating diffusion region and shortens the time required for the photocharges to reach the floating diffusion region. This solid-state image sensor is also characterized in that a potential gradient is formed in the direction from the circumference of the light-receiving surface to the center thereof (i.e. to the floating diffusion region) so that the photocharges created in the photodiode can easily gather at the center of the light-receiving surface.
In the previously described solid-state image sensor, since the floating diffusion region, which collects electrons, is located at the center of the light-receiving surface, it is normally necessary to provide a metallic or similar wire for connecting the floating diffusion region to another region outside the light-receiving surface so as to send electric charges through this wire to a circuit in the subsequent stage. However, this wire is a capacitive load, which lowers the charge-to-voltage conversion gain of the floating diffusion region and decreases its sensitivity. Additionally, the image sensor according to Patent Document 4, in which the channel width monotonously decreases as the distance from the center increases, has the problem that, if a P-well/N-sub structure is adopted for some reason (e.g. in order to use a vertical overflow drain or suppress crosstalk), the photocharges generated in an area where no channel is present will flow toward the substrate, which results in a decrease in the sensitivity.